Despite many of good electrical, mechanical and optical properties of carbon nanotubes (CNTs), applications for CNT's are still generally limited. One of the main challenges faced for the development of CNT-based materials is the dispersion and stabilization of CNTs in a solvent media. CNTs generally form bundles due to strong van der Waals interactions between the respective nanotubes. Known methods to disperse CNT's into solvents and polymer matrices include direct mixing, chemical functionalization and third component-assisted dispersion. Each of these methods provide certain advantages and certain disadvantages.
Exfoliation, using mechanical forces such as sonication, although simple and cost effective, generally results in poor quality dispersions. Nanotubes dispersed in this manner quickly (e.g. within several minutes) precipitate from suspension or solution when the mechanical force stops.
Chemical modification and functionalization of CNTs generally involves treatment with a strong oxidizing acid, such as nitric acid, to form functional groups, such as carboxylic acid groups on the walls of the nanotube. Modification is obtained by breaking carbon-carbon double bonds of the CNTs and turning them into functional groups, such as COOH groups in the case of strong acid treatment. In this manner CNTs can be made sufficiently polar to become water soluble or, upon modification of the carboxylic acid groups, soluble in certain organic liquids. Although chemical modification is generally effective as a dispersion method, such treatment inherently disrupts the long range nt conjugation of the nanotube, resulting in diminished electrical conductivity, mechanical strength, and/or a degradation in other properties.
Third component-assisted dispersion is currently considered to be the most effective approach. Surfactants molecules such as sodium dodecyl sulfate (SDS), water soluble block copolymers, DNA, proteins, and polyelectrolytes can be used as the third component. These third components are primarily aimed at forming CNT dispersions in aqueous solutions. Recently, conjugated conducting polymers have attracted much attention as dispersants for CNTs. Conjugated polymers are polymers with SP2 hybridized linear carbon back bones. A wide variety of conjugated conducting polymers such as poly(m-phenylene vinylene), poly(3-alkylthiophene), and poly(arylene ethynylene) have been shown to be effective for dispersing CNTs. The dispersion effect is attributed to the π-π interactions between the conjugated conducting polymers and the CNTs. However, such π-π interactions limit the solubility of the conjugated polymers in solvents and such CNT-conjugated polymers are generally immiscible with other polymer systems.
Hence, there remains a need for stable CNT dispersions or solutions, particularly in non-aqueous solutions, where the desired properties (e.g. electrical and mechanical) of the nanotubes are not compromised by reactions that can alter their surfaces, and where little or no aggregation of the nanotubes are present in the dispersion, and the dispersion remains stable for significant lengths of time, such as weeks or months. Stable CNT dispersions or solutions would also be useful to form composite materials and to develop new coatings for use in a variety of technologies and applications.